AC permanent magnet synchronous motors are an important type of AC synchronous machines which can operate at predetermined constant speeds for producing mechanical actuation, and such motors have been widely used in a variety of industry environments. The conventional AC permanent magnet synchronous motors are normally constructed with a stator having a primary winding for generating a rotating magnetic field to provide rotation torque under energization of AC power. The AC permanent magnet motor also includes a rotor having a central shaft and a stack of laminations mounted on the shaft which hold a plurality of conductive bars (i.e., squirrel cage bars) for starting the rotation of the rotor and a plurality of permanent magnets for producing an even number of magnetic poles on the periphery of the rotor to lock the rotor at particular synchronous speeds.
Three important considerations for any motor are power factor, efficiency and maximum speed limit. A spinning rotor undergoes centrifugal forces which tend to force the rotating rotor away from its axis. This force becomes bigger with an increase of rotor speed so that the maximum speed of a rotor is limited. Therefore, strength is a very important parameter to high speed motors. In addition, the magnetic field created by the permanent magnets secured by the rotor laminations and the rotating magnetic field produced by the primary winding on the stator characteristically have flux leakage through the laminations. As a result, the power factor, efficiency and other parameters, such as pull in and pull out torques of the motor are very much affected. These parameters are also very important to a high speed motor. Over the years, various rotor laminations have been designed to achieve a high strength for resisting centrifugal forces and at the same time for reducing the flux leakage for enhancing the motor's power factor, efficiency and maximum speed limit.
A typical example of such a rotor lamination is shown in FIG. 1A, which is disclosed by U.S. Pat. No. 4,139,790 issued on Feb. 13, 1979 to Charles R. Steen. The rotor lamination illustrated by the upper part of FIG. 1A has an inner portion 21 and an outer portion 22 connected by bridges 25 between conductive bar slots 23 and the periphery of the rotor lamination. The rotation magnetic field .phi.1 is created by the stator primary windings for providing a basis rotation torque. The magnetic flux [.phi.5] is generated by the permanent magnets 24 for synchronous operation. The bridges 25 are normally very narrow in order to achieve a high power factor and efficiency. If the bridges 25 are widened, the leakage flux [.phi.2] from the permanent magnets, and particularly, the leakage flux .phi.4 from the stator windings will consequently increase to result in more power loss. Because, the interconnections between the two portions 21 and 22 of the lamination are very weak, the maximum speed is limited. In order to obtain high strength for the rotor without increasing the width of the bridges, non-magnetic laminations are utilized between the magnetically permeable laminations. This structure requires the use of more expensive material such as stainless steel which increases the rotor's longitudinal dimension. On the other hand, since non-magnetic laminations extend from the shaft to the outside of the rotor, flux leakage occurs along the entire magnetic width of each non-magnetic lamination. A modification to magnetically permeable laminations which serves to strengthen the rotor lamination is to provide bridges 27 at the end of each permanent magnet and bridges 28 as shown in FIG. 1A. However, such bridges 27 will make the manufacture of the lamination very difficult. For example, since the relative position of each permanent magnet 24 to a flux leakage barrier 26, connected to a bar slot 23, is fixed and must be very accurate in order to obtain a narrow bridges 27, it is hard to stamp the permanent magnet slots by using simple tools. Further, the bridges 27 will be easily broken during installation of the permanent magnets 24 into the magnet slots. Furthermore, such bridges 27 enable more flux leakage (.phi.3) from permanent magnets to occur which will affect the uniformity of the magnetic field and flux density of the field.
Other examples of prior rotors and rotor laminations are disclosed in U.S. Pat. No. 4,358,697 issued on Nov. 9, 1982 to Joseph C. Liu, et al., U.S. Pat. No. 4,358,696 issued on Nov. 9, 1982 to Joseph C. Liu et al., U.S. Pat. No. 4,469,970, issued on Sept. 4, 1984 to Thomas W. Neumann, U.S. Pat. No. 4,476,408 issued on Oct. 9, 1984 to Vernon B. Honsinger, U.S. Pat. No. 4,525,925 issued on July 2, 1985 to Donald W. Jones and U.S. Pat. No. 4,568,846 issued on Feb. 4, 1986 to Shailesh C. Kapedia.
The rotors and rotor limitations disclosed in the above patents either arrange magnets in complicated patterns or use special strength laminations or special strengthening material in the laminations. These approaches make the rotor laminations more difficult to manufacture and more expensive. Additionally, all of the prior art rotor laminations have flux leakage (.phi.4) from the rotating magnetic field which passes through the bridges between the conductive bar slots and the periphery of the rotor lamination, as shown (for example) in the upper portion of FIG. 1A.
FIG. 1B depicts a rotor lamination manufactured by Reuland Electric Inc. The rotor lamination utilizes openings 29 between the conductive bar slots 23 and the periphery of the rotor lamination for eliminating flux leakage (.phi.4) as shown in FIG. 1A. However, this product has a large amount of flux leakage (.phi.2) and (.phi.3).
The present invention overcomes the shortcomings of such prior art rotor laminations and provides an economically manufacturable rotor lamination for an AC permanent magnet synchronous motor.